A downhole well is drilled in stages, and casing is run down the inside of the well to support the raw sides once certain depth has been reached. The casing is cemented in place and the process is repeated to the next depth. At each subsequent stage, smaller diameter casing is used.
Alternatively, a liner string is used. In comparison to a casing string, which runs from the well head down the complete length of the well, a liner string is hung from a casing-mounted liner hanger before being cemented in place.
After the casing or liner is cemented in place, and in preparation for the completion phase of the well, drilling fluid or “mud” is circulated out of the well and replaced by substantially solids-free brine. Although solids in the drilling fluid may be necessary or desirable when drilling the well, solids are generally undesirable in the completion phase; the solid particles may clog the producing formation and reduce the flow of the well. The “clean-out” phase conventionally involves circulating “pills” of surfactants into the well and filtration of the circulating fluid to assist in removing solids and clean out the well.
Oil-based drilling muds tend to leave a sticky residue on the wall of the casing. This occurs because of natural oil-wet water-wet separation and even with a good cleaning velocity in a circulating fluid, a small boundary layer by the wall of the casing will be present and this layer is barely moving. Enhanced cleaning may be achieved by directing a treatment fluid through nozzles onto a target area. A controlled rotary motion may be used to achieve complete circumferential treatment. Alternatively, the residue may be physically dislodged from the casing wall using brushes or scrapers.
When commercial production of a well is no longer viable, the well must be decommissioned. For an offshore well, some of the liner and casing is cut and retrieved, the subsea production tree is cut and the well is then plugged. Conventionally, casing is cut by attaching a cutting tool to the end of a work string and running the tool down into the casing.
There are various tools available for downhole cutting and severing of casing strings. These tools utilise multiple extendable arms or blades which rotate outwardly to contact the casing. The work string and cutting tool is rotated causing cutting of the casing string. In some cases the cutting tool is motor driven. For example, US2011220357 (A1) describes a method for milling a tubular cemented in a wellbore including deploying a work string-mounted bottom hole assembly (BHA) into the wellbore through the tubular, the BHA comprising a window mill with extending arms. The method further comprises rotating the work string, extending the arms of the window mill, and radially cutting through the tubular.